In Japanese Patent Nos. 87215591 and 8798685, and European Patent No. 225552, there are described a number of methods for preparing pyrido benzoxazine derivatives having antibacterial activity. Further, similar antibacterially active compounds to those supra patent and their preparation methods are disclosed in Korean Patent Publication No. 84-2141 and Korean Patent Laid-Open Publication Nos. 92-22050, 93-4309, 93-4310, 93-10027, 93-10041 and 93-12787.